One known type of information storage device is a disk drive device that uses magnetic media to store data and a movable read/write head that is positioned over the media to selectively read from or write to the disk.
Consumers are constantly desiring greater storage capacity for such disk drive devices, as well as faster and more accurate reading and writing operations. Thus, disk drive manufacturers have continued to develop higher capacity disk drives by, for example, increasing the density of the information tracks on the disks by using a narrower track width and/or a narrower track pitch. However, each increase in track density requires that the disk drive device have a corresponding increase in the positional control of the read/write head in order to enable quick and accurate reading and writing operations using the higher density disks. As track density increases, it becomes more and more difficult using known technology to quickly and accurately position the read/write head over the desired information tracks on the storage media. Thus, disk drive manufacturers are constantly seeking ways to improve the positional control of the read/write head in order to take advantage of the continual increases in track density.
One approach that has been effectively used by disk drive manufacturers to improve the positional control of read/write heads for higher density disks is to employ a secondary actuator, known as a micro-actuator, that works in conjunction with a primary actuator to enable quick and accurate positional control for the read/write head. Disk drives that incorporate a micro-actuator are known as dual-stage actuator systems.
Various dual-stage actuator systems have been developed in the past for the purpose of increasing the access speed and fine tuning the position of the read/write head over the desired tracks on high density storage media. Such dual-stage actuator systems typically include a primary voice-coil motor (VCM) actuator and a secondary micro-actuator, such as a PZT element micro-actuator. The VCM actuator is controlled by a servo control system that rotates the actuator arm that supports the read/write head to position the read/write head over the desired information track on the storage media. The PZT element micro-actuator is used in conjunction with the VCM actuator for the purpose of increasing the positioning access speed and fine tuning the exact position of the read/write head over the desired track. Thus, the VCM actuator typically makes larger adjustments to the position of the read/write head, while the PZT element micro-actuator makes smaller adjustments that fine tune the position of the read/write head relative to the storage media. In conjunction, the VCM actuator and the PZT element micro-actuator enable information to be efficiently and accurately written to and read from high density storage media.
One known type of micro-actuator incorporates PZT elements for causing fine positional adjustments of the read/write head. Such PZT micro-actuators include associated electronics that are operable to excite the PZT elements on the micro-actuator to selectively cause expansion or contraction thereof. The PZT micro-actuator is configured such that expansion or contraction of the PZT elements causes movement of the micro-actuator which, in turn, causes movement of the read/write head. This movement is used to make faster and finer adjustments to the position of the read/write head, as compared to a disk drive unit that uses only a VCM actuator.
In a conventional disk drive unit, a magnetic disk is mounted on a spindle motor for spinning the disk. A voice coil motor arm carries a head gimbal assembly (HGA) that includes a micro-actuator with a slider incorporating a read/write head. A voice-coil motor (VCM) is provided for controlling the motion of the motor arm and, in turn, controlling the slider to move from track to track across the surface of the disk, thereby enabling the read/write head to read data from or write data to the disk. In operation, a lift force is generated by the aerodynamic interaction between the slider, incorporating the read/write transducer, and the spinning magnetic disk. The lift force is opposed by equal and opposite spring forces applied by a suspension of the HGA such that a predetermined flying height above the surface of the spinning disk is maintained over a full radial stroke of the motor arm.
Because of the inherent tolerances of the VCM and the head suspension assembly, the slider cannot achieve quick and fine position control which adversely impacts the ability of the read/write head to accurately read data from and write data to the disk. As a result, a PZT micro-actuator, as described above, is typically provided in order to improve the positional control of the slider and the read/write head. More particularly, the PZT micro-actuator corrects the displacement of the slider on a much smaller scale, as compared to the VCM, in order to compensate for the resonance tolerance of the VCM and/or head suspension assembly. The PZT micro-actuator enables, for example, the use of a smaller recording track pitch, and can increase the “tracks-per-inch” (TPI) value by 50% for the disk drive unit, as well as provide an advantageous reduction in the head seeking and settling time. Thus, the PZT micro-actuator enables the disk drive device to have a significant increase in the surface recording density of the information storage disks used therein.
A hard disk drive, particularly a micro drive of the type to which the instant invention is directed, generally benefits from a compact size as well as a stable actuator. Many hard disk drives include only one coil in the voice coil motor (VCM) system, which may be enough to successfully operate. But with a single coil, it is difficult provide an advantageous relationship between the center of gravity and the center of force. Moreover, using an asymmetrical method of driving forces, the device may be unstable and unsuitable for high speed periodic rotation applications, such as in magnetic and/or magnetic/optical disk drive devices.
In the data storage field, the coil mass is generally used to offset the head arm mass to assist in balancing the actuator. Thus, the coil mass is typically asymmetrical. Thus, in high-end products, it is not uncommon to add a second actuator (such as the PZT micro-actuator described above) in the hard disk drive in order to minimize the frequency of off-track errors.
A prior art hard disk drive (HDD) typically contains an actuator for positioning the magnetic head over the upper and lower surfaces of the disk and for carrying the rotating arm back and forth (thus facilitating reading data from and writing data to the magnetic hard disk). There are generally two categories of voice coil motor designs: moving coil (MC) and moving magnet (MM). Of these two types, the moving magnet (MM) design is generally more prevalent in hard disk drives available on the market today.
FIG. 1 depicts a typical prior art micro drive incorporating a moving coil (MC)-type voice coil motor (VCM) as the primary actuator. In the disk drive device of FIG. 1, there are three basic components of the voice coil motor (VCM): a magnet 1, a yoke 2, and a movable coil 3. The movable coil 3 can be excited by a driving current. When the movable coil 3 is excited, arm 7 and head gimbal assembly 6 are rotated around shaft 8 which is fixed relative to base 10. As a result of a potential dynamic balance effect and possible eccentricity of spindle motor 9, the magnetic head 4 may, for example, be required to adjust its position in order to correctly read data from or correctly write data to an oscillating or fluctuating disk 5.
FIG. 2 illustrates a conventional hard disk drive—in particular, a micro drive—of the moving magnet (MM)-type. In the device of FIG. 2, magnet 1′ and yoke 2′ are movable, and the coil assembly 3′ is fixed to base 10. In such as device, the assembly process may be simplified by changing the coil type from a winding formation to an integrated flex loop 3′. Based on the Faraday principle, a Lorentz force can be generated by the energized coils and permanent magnets. If the coils are constrained and fixed on base 10, the interactive force will make the magnets move and rotate around shaft 8 together with arm 7 and head gimbal assembly 6. The magnetic head 4 can thus be operated by servo control to adjust its relative position on disk 5. Accordingly, a symmetric VCM structure is similar to the structure of the spindle motor 9. Although the MM-type VCM may have advantages relating to size reduction and low-cost, there may be a risk of magnet contamination, which is not welcome among manufacturers of disk drive devices.
In a conventional system, a MM-type VCM generally requires a smaller footprint in a hard disk drive (HDD) than a MC-type. In general terms, both a MM-type and a MC-type VCM will use a pair of driving forces to generate a relative rotation. As the size of a hard disk drive decreases, it becomes more difficult to employ a symmetric design for the VCM. Thus, while symmetric designs have been developed, they have not been widely adopted in the industry due, at least in part, to the various problems discussed herein.
FIG. 3 illustrates a conventional MC-type VCM.
In addition to magnet 1 and bottom yoke 2a, the VCM contains a top yoke 2b, which may assist in increasing the magnetic flux density. Normally, there are two air gaps in a MC-type VCM. One exists between magnet 1 and coil 3, while the other exists between coil 3 and top yoke 2b. In a MM-type VCM, however, there is generally only one air gap. The present invention can assist in minimizing the air gap(s) in voice coil motor assemblies.
Although a MM-type VCM may be compact, thin and inexpensive, they typically are heavier in weight. This heavy weight can cause the moment of inertia to be greater than with the traditional MC-type VCM. In addition, if there is permeable material under the integrated flex loop, the interactive force applied on the magnet may result in a great deal of friction. Friction, in turn, may cause a MM-type VCM to operate more slowly than a MC-type VCM.
The use of an asymmetric driving method, furthermore, is not preferable. Some designers, for example, prefer to have a symmetric layout in a VCM assembly. However, under presently known VCM designs, its is difficult to introduce an additional coil in the VCM assembly. Although some symmetric designs have been proposed, they do not overcome problems relating to counterforces and also have not been compact and efficient enough for widespread implementation.
The instant invention is intended to solve one or more of the above-described problems with prior art VCM designs, and to provide a symmetric and more compact VCM design, which is particularly useful in a micro drive.